On rooting... All rooting is is gaining administrative privileges for your device. It allows you to access and write to the system partition. Because this is a security risk, many apps that require advanced security (like banking, finance, medical, etc.) may not work on a rooted device. Just saying.
By having root privileges, you can links2sd but I'm not sure if it works with any android phone after 5.0 since partitions are allocated differently. And BTW, moving apps to SD is not the same as adoptable storage. Links to SD really just formats a partition on your SD card in the same format as the internal storage and then creates symlinks that point to the SD where the app thinks they should normally reside. Adoptable storage extends the internal user partition to the SD card making it indistinguishable to the system.
Sounds great, right? Why not do that? Well, here's why. Micro SD cards are notoriously flaky when stressed under the load of continual read/writes from running apps directly from them. Because they are part of the user partition, when they fail (not if) they can take everything with them, even the files on internal memory. Plus, when used as adoptable storage, the card is encrypted and should never be removed from the phone ... ever .. unless you are going to perform a factory reset and format the card. SD cards are fine for storing anything that do not impact the system and you can backup like photos or media files.
Here's how Google backup/restore works. When you check that box to backup your apps and settings what you are really doing is backing up ONLY settings -- and this is important -- NO DATA. What does that mean? Well, if you ever have to restore your phone from the cloud, what restore will do is look at the list of apps the play store thinks you had on the device and reload them, in the latest version, from the play store. It will then attempt to overwrite the default settings with the ones it backed up from your phone. 98% of the time that's good enough, but if you have local data associated with the app, Google does NOT back that up, unless you tell it to specifically with Drive sync. So let's say you are playing a game that maintains your account in the cloud and one that saves game data locally. If you drop your phone in the pool and have to get a new one, when you restore, the games will load and the first game will be fine and you can pick up where you left off. The second game will most likely start you back at square one, but with the same credentials.
You are discovering the drawbacks of going cheap with Android. While it's certainly possible to use the device and many people do every day, you will never have the Android Flagship experience on a $200 phone.
